1. Field of the Invention
This invention relates to an active distance measuring apparatus for camera, and more particularly to distance measurement in a wide visual field.
2. Description of the Related Art
As shown in FIG. 1, the conventional distance measuring apparatus is constructed by light projection and reception lenses 101 and 102 having two optical axes which are separated by a preset base length (S), a light projection IRED (near infrared light emitting diode) 103, and a PSD (position sensing device) 104 for light reception having a length t.
The PSD 104 is disposed such that the end face thereof may be set at a distance a from the optical axis 102a of the reception lens 102. In this arrangement, the centroid of a reflected spot 106 from a subject 105 is set on a position at a distance of [(Sf.sub.j /D)+a] from the end face of the PSD 104 where D indicates a distance to the subject and f.sub.j indicates the focal distance of the light reception lens 102. Since the PSD 104 generates a current output corresponding to the position of the reflected spot 106, the following equation (1) can be obtained if one end portion of the PSD 104 which lies near the light projection lens 101 is used as a first electrode (1 ch), the other end portion thereof is used as a second electrode (2 ch), and output variation currents caused in the respective portions when the projection IRED 103 has emitted light are i.sub.1 and i.sub.2. ##EQU1##
The following equation (2) can be obtained by rewriting the equation (1). ##EQU2##
Therefore, the reciprocal 1/D of the distance to the subject can be obtained by calculating [i.sub.2 /(i.sub.1 +i.sub.2)] by use of the currents i.sub.1 and i.sub.2 according to the equation (2).
In the example of FIG. 1, the distance to only one subject 105 disposed at the center can be measured. However, it is possible to use a plurality of IREDs 103 having different optical axes as in a three-point distance measuring device as shown in FIG. 2A which is also similar to that which is disclosed in published Examined Japanese Patent Application (PEJPA) NO. 3-2245, for example.
When the three-point distance measurement is effected as shown in FIG. 2A, no problem will occur if the entire portion of the spot of the IRED 103 is projected on the subject, but if part of the spot is not projected onto the subject as shown in FIG. 2B (that is, a so-called spot eclipse occurs), the centroid of the reflected spot image on the PSD 104 is deviated from that obtained when the entire spot is reflected. As a result, the distance to the subject will be erroneously measured.
As a measure for coping with the spot eclipse, a binocular AF (automatic focusing) method as shown in FIG. 3 is proposed.
The AF is effected by symmetrically disposing two light reception lenses 102L and 102R with respect to the optical axis 101a of the projection lens 101 at a distance of base length S from the optical axis 101a and symmetrically disposing two PSDs 104 (PSD-L 104L and PSD-R 104R) with respect to the optical axis 101a at the distance of base length S from the optical axis 101a. In this case, output currents of PSD-L and PSD-R are respectively indicated by i.sub.1L, i.sub.2L, i.sub.1R and i.sub.2R. If the spot eclipse occurs and the centroid of the subject 105 is deviated from the optical axis 101a of the projection lens 101 by x, the relations between the reflected spot positions of the right and left PSDs and the output currents can be expressed by the following equations (3). ##EQU3##
The following equations (4) can be obtained by connecting the 1-ch portions and 2-ch portions of the right and left PSDs together, and setting the respective output currents to i.sub.1, i.sub.2. EQU i.sub.1 =i.sub.1L +i.sub.1R, i.sub.2 =i.sub.2L +i.sub.2R ( 4)
In this case, since the right and left PSDs are disposed to detect the same subject 105, the following equation (5) can be obtained. EQU i.sub.1L +i.sub.2L =i.sub.1R +i.sub.2R ( 5)
The following equation (6) can be derived based on the equations (3), (4) and (5) and the distance D to the subject can be determined independently from .DELTA.x as shown in FIG. (6). ##EQU4##
However, in the three-point AF (FIG. 2A) described before, when the subject 105 lies in a position other than the three points, for example, a position between the beams, the distance cannot be measured. That is, when none of the projected beams are applied to the subject 105, a so-called spot eclipse occurs and the distance to the subject may be erroneously determined.
In contrast, the binocular AF (FIG. 3) described before can effectively cope with the spot eclipse but it has a defect that the distance cannot be measured in a wide range. That is, if the projection angle of the IRED 103 is increased to measure the distance in a wide range in the AF shown in FIG. 3, the distance to the subject may be erroneously determined by the influence due to something (such as a wall behind a human) other than the main subject.